Stabilizing silver halide emulsions with cadmium bromide



United States Patent 3,488 709 STABILIZING SILVER HALIDE EMULSIONS WITH CADMIUM BROMIDE Nelson R. Sidebotham, Rochester, N.Y., assignor to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey No Drawing. Filed Dec. 2, 1966, Ser. No. 598,615 int. Cl. G03c 1/08, 1/28 US. Cl. 96-110 13 Claims ABSTRACT OF THE DISCLOSURE Silver halide emulsions comprising (1) silver halide grains formed in the presence of a rhodium salt and (2) a stabilizing concentration of cadmium bromide.

This invention relates to photographic materials, their preparation and use. One aspect of this invention relates to a process for preparing silver halide emulsions by precipitation in the presence of a rhodium salt and stabilizing the emulsions with cadmium bromide. Another aspect of the invention relates to the silver halide emulsions and photographic elements containing these emulsions which are prepared in accordance with the process described herein.

When silver halide emulsions are prepared by precipitation in the presence of small quantities of a rhodium salt, it is known that the contrast is considerably higher than that obtained with emulsions precipitated in the absence of a rhodium salt. This high-contrast advantage, however,

is sometimes lost during storage of the material. It would be very desirable to obtain an effective stabilizer for silver halide emulsions prepared by precipitation in the presence of a rhodium salt which would reduce the loss of contrast upon storage. Another disadvantage of emulsions of this type is that they tend to gain speed, i.e., sensitivity, upon storage. It would be very desirable to reduce the speed change upon storage of such silver halide emulsions. The instant invention provides a solution to these problems.

Accordingly, it is an object of this invention to provide a process for preparing silver halide emulsions by precipitation in the presence of a rhodium salt which are stabilized against loss of contrast and speed change upon storage. Another object of this invention is to provide a process for preparing silver halide emulsions in the presence of a rhodium salt and a silver halide solvent which are stabilized against loss of contrast and speed change upon storage. A further object of the invention is to provide silver halide emulsions and photographic elements containing these emulsions which are stabilized against loss of contrast and speed change upon storage. Still another object is to provide silver halide emulsions with a synergistically higher contrast. Other objects will become apparent hereinafter.

I have discovered that silver halide emulsions prepared by precipitation in the presence of a rhodium salt to increase the contrast can be stabilized against loss of contrast and speed change upon storage by adding to the silver halide emulsion after it has been prepared, Cadmium bromide in a stabilizing amount. Further, silver halide emulsions which have been prepared in the presence of a rhodium salt and a silver halide solvent can also be stabilized in the same manner. The rhodium salt-cadmium bromide combination also performs as a synergistic com-.

bination in that the contrast obtained is higher than that obtainable with either constituent alone.

Rhodium salts are known to be useful to increase the contrast of a silver halide emulsion. This is disclosed, for example, in British Patent 775,197 and Glafkides, Photo- Patented .Jan. 6, 1970 ice be any water-soluble rhodium salt useful for the intended, purpose and includes those disclosed in the above refer-f ences and are employed in the same manner. Typical useful rhodium salts are, for example, rhodium chloride, rhodium trichloride, rhodium ammonium chloride, etc. The rhodium salts can be employed in any concentration which is effective for the intended purpose. Especially good results were found when the concentration was from about 0.01 to about 0.25 milligram per mole of silver halide.

Cadmium bromide was unexpectedly found to stabilize silver halide emulsions precipitated in the presence of rhodium salts in that it reduces the loss of contrast and speed change upon storage. Cadmium bromide also acts to increase the contrast in a synergistic manner. Other closely related compounds such as potassium bromide and cadmium chloride did not have these properties as will be shown hereinafter.

The cadmium bromide can be employed in my invention in any concentration which will stabilize the silver halide emulsions. A particularly useful concentration range was found to be from about 20 to about 60 grams per mole of silver halide. The cadmium bromide is added to the silver halide emulsion after the formation of the silver halide grains.

Methods for preparing silver halide emulsions are well known in the art as shown, for example, by the Glafkides reference mentioned above. My invention can be employed with any technique for making silver halide emulsions. For example, the double-jet technique can be employed where solutions of potassium halide and silver nitrate are simultaneously run into an aqueous gelatin solution at 150 F., or the single-jet technique can be employed where silver nitrate is added to an aqueous gelatin solution already containing potassium bromide.

Cadmium salts have been employed in the photographic field for many purposes. The Glafkides reference mentioned above discloses on page 351 that cadmium chloride may be used to increase the contrast in a silver halide emulsion. This material is employedin the same manner as a rhodium salt, however, and is added to the silver halide emulsion during precipitation. In my invention, c'adrnium bromide is employed and is added to the silver halide emulsion, prepared in the presence of a rho dium salt, after the emulsion is formed to stabilize the emulsion against loss of contrast and speed change upon storage. The cadmium bromide also synergistically increases the contrast.

US. Patents 2,950,972 and 3,000,739 employ cadmium bromide during the precipitation of silver halide emulsions for various purposes. These patents do not disclose anything about adding cadmium bromide to the silver halide emulsion after it is formed, however, and do not teach anything concerning stabilizing the emulsion against loss of contrast and speed change upon storage or that rho dium salts should be added to increase contrast or that cadmium bromide will synergistically increase the contrast.

US. Patent 2,8 39,405 discloses various antifoggants for silver halide emulsions including various inorganic acid salts. N0 bromide compounds are specifically mentioned, however, and cadmium chloride and potassium bromide,

' which are closely related to cadmium bromide, do not rhodium salts are disclosed in this patent as sensitizers, these are ordinarily not present during precipitation of the silver halide grains but are added after grain formation.

The following examples will illustrate the invention but are not to be construed to limit it in any way.

Example 1 A silver bromoiodide (99:1) emulsion was prepared using the double-jet technique. An aqueous solution of potassium bromide and potassium iodide and another aqueous solution of silver nitrate were simultaneously added to an aqueous gelation solution at 150 F. with rapid stirring. A precipitate of silver bromoiodide was formed which was then washed to remove insoluble salts. The emulsion was then sulfur and gold sensitized in the conventional manner. A similar emulsion was prepared in the same manner but with the potassium bromide and potassium iodide solution containing 0.15 milligram of rhodium ammonium chloride per mole of silver halide being formed. Cadmium bromide (40 grams per silver mole) was then added to portions of each emulsion after preparation. The emulsions were then coated on a support having a gelatin layer containing 67 milligrams per square foot of hydroquinone and 17 milligrams per square foot of 1-phenyl-3-pyrazolidone. The coated elements were then exposed on an intensity scale sensitometer and processed with an alkaline activator solution and then with a thiocyanate stabilizing solution in the conventional manner. The following results were obtained:

Contrast The above data indicate that there is a synergistic effect in using the rhodium salt in combination with the cadmium bromide in that the initial contrast is higher than the contrast obtained with either one alone. The above data also indicate that there is a smaller percentage loss of contrast upon incubation and smaller speed change upon incubation when the cadmium bromide is used with an emulsion precipitated in the presence of a rhodium salt. Thus, more of the higher contrast obtained by using a rhodium salt can be retained upon incubation along with a relatively smallspeed change when the instant invention is employed.

Similar results are obtained when a silver halide solvent is present during the formation of the silver halide grains or when the silver halide grains are digested after formation in the presence of a silver halide solvent. The silver halide solvents are water-soluble compounds such as the thiocycanates and preferably organic thioethers of the type disclosed in US. Patent 3,271,157 of McBride, granted Sept. 6, 1966. The contrast is not substantially affected by such silver halide solvents and while the initial speed is higher, the speed change upon incubation is approximately the same.

Example 2 An emulsion was prepared in the same manner as in Example 1 with rhodium ammonium chloride being present during precipitation. To several portions of the emulsion after preparation were added cadmium bromide (40 grams per silver mole), cadmium chloride (19 grams per silver mole) and potassium bromide (27 grams per silver mole). The emulsions were then coated on a support similar to that of Example 1, exposed and processed in the same manner. The following results were obtained:

It is thus seen that closely related compounds to cadmium bromide do not give resluts as good as those obtained using cadmium bromide. The use of cadmium bromide gives the highest initial contrast, the lowest percentage change in contrast upon incubation and the smallest speed change upon incubation. The combination of these improved properties provides a stability criterion for high contrast elements which is very significant in commercial applications.

The silver halide emulsions of the instant invention can be sensitized using any of the well-known techniques in emulsion making, for example, by digesting with naturally active gelatin or various sulfur, selenium, tellurium compounds and/ or gold compounds. The emulsions can be sensitized with salts of noble metals of Group VIII of the Periodic Table which have an atomic weight greater than 100. The emulsions can also contain speed increasing addenda such as quaternary ammonium salts, polyethylene glycols or thioethers.

The silver halide emulsions of the instant invention can also contain conventional addenda such as gelatin plasticizers, coating aids, antifoggants and hardeners such as aldehyde hardeners, e.g., formaldehyde, mucochloric acid, glutaraldehyde bis(sodium bisulfite), maleic dialdehyde; aziridines, dioxane derivatives and oxypolysaccharides. The addenda can be added to the emulsions before or after sensitizing dyes, if any, are used. Sensitizing dyes useful in sensitizing such emulsions are described, for example, in US. Patents 2,526,632 and 2,503,776. Spectral sensitizers which can be used are the cyanines, merocyanines, complex (trinuclear) cyanines, complex (trinuclear) merocyanines, styryls and hemicyanines. Developing agents can also be incorporated into the silver halide emulsions if desired or be contained in a separate underlayer. Various silver salts can be used as the sensitive salt such as silver bromide, silver iodide, silver chloride, or mixed silver halides such as silver chlorobromide or silver bromoiodide. Particularly useful silver bromoiodides are those containing at least about mole percent bromide. The silver halides used can be those which form latent images predominantly on the surface of the silver halide grains.

The silver halide emulsions of the instant invention can contain any of the hydrophilic water-permeable binding materials suitable for this purpose. Suitable materials include gelatin, colloidal albumin, polyvinyl compounds, cellulose derivatives, acrylamide polymers, etc. Mixtures of these binding agents can also be used. The binding agents for the emulsion layer of the photographic element can also contain dispersed polymerized vinyl compounds. Such compounds are disclosed, for example, in US. Patents 3,142,568; 3,193,386; 3,062,674; and 3,220,844, and include the water-insoluble polymers of alkyl acrylates and methacrylates, acrylic acid, sulfoalkyl acrylates or methacrylates and the like.

The silver halide emulsions of the instant invention can be coated on a wide variety of supports. Typical supports are cellulose nitrate film, cellulose ester film, polyvinyl acetal film, polystyrene film, poly(ethylene terephthalate) film and related films or resinous materials as well as glass, paper, metal and the like. Supports such as paper which are coated with ot-olefin polymers, particularly polymers of a-olefins containing two or more carbon atoms, as exemplified by polyethylene, polypropylene, ethylenebutene copolymers and the like can also be employed.

The silver halide emulsions of the instant invention have a very high contrast and can advantageously be used as projection-speed emulsions and are particularly useful when coated on a support for use as photographic printing papers such as enlarging papers.

Although the invention has been described in considerable detail with particular reference to certain preferred embodiments thereof, variations and modifications can be effected within the spirit and scope of the invention as described hereinbefore and as defined in the appended claims.

I claim:

1. A process comprising precipitating silver halide grains of a photographic emulsion in the presence of a rhodium salt and adding to said emulsion, a stabilizing concentration of cadmium bromide.

2. The process of claim 1 wherein a silver halide solvent is present during the preparation of said emulsion.

3. The process of claim 1 wherein the concentration of said rhodium salt is from about 0.01 to about 0.25 milligram per mole of silver and the concentration of said cadmium bromide is from about 20 to about 60 grams per mole of silver.

4. The process of claim 3 wherein at least 90 mole percent of said halide is bromide,

5. A silver halide emulsion comprising (1) silver halide grains formed in the presence of a rhodium saltan i (2) a stabilizing concentration of cadmium bromide.

6. A silver halide emulsion according to claim 5 wherein said silver halide grains are formed in the presence of a silver halide solvent.

7. A silver halide emulsion according to claim 5 wherein the concentration of said cadmium bromide is from about 20 to about grams per mole of silver.

8. A silver halide composition according to claim 5 wherein said silver halide grains are formed in the presence of about 0.01 to about 0.25 milligram of a rhodium salt per mole of silver formed.

9. A silver halide composition according to claim 5 wherein at least mole percent of said halide is bromide.

10. A photographic element comprising a support coated with the silver halide emulsion of claim 5.

11. A photographic element comprising a support coated with the silver halide emulsion of claim 6. V

12. A photographic element comprising a support coated with the silver halide emulsion of claim 7.

13. A photographic element comprising a support coated with the silver halide emulsion of claim 9.

References Cited UNITED STATES PATENTS 2,717,833 9/1955 Wark 96l08 2,839,405 6/1958 Jones 96-110 XR NORMAN G. TORCHIN, Primary Examiner RICHARD E. FIGHTER, Assistant Examiner US. Cl. X.R. 96-108 

